Dose Modeling and Statistical Assessment of Hot Spots for Decommissioning Applications

A primary goal of this research was to develop a technically defensible approach for modeling the receptor dose due to smaller "hot spots" of residual radioactivity. Nearly 700 combinations of environmental pathways, radionuclides and hot spot sizes were evaluated in this work. The hot spot sizes studied ranged from 0.01 m² to 10 m², and included both building and land area exposure pathways. Dose modeling codes RESRAD, RESRAD-BUILD, and MicroShield were used to assess hot spot doses and develop pathway-specific area factors for eleven radionuclides. These area factors are proposed for use within the existing Multiagency Radiation Survey and Site Investigation Manual (MARSSIM) context of final status survey design and implementation. The research identified pathways that are particularly "hot spot sensitive"—i.e., particularly sensitive to changes in the areal size of the contaminated area. The external radiation pathway was the most hot spot sensitive for eight of the eleven radionuclides studied. These area factors were evaluated both when the receptor was located directly on the soil hot spot and ranged from 6.6 to 11.4 for 1 m² hot spot; and ranged from 650 to 785 when the receptor was located 6 m from the 1 m² hot spot. The external radiation pathway was also the most sensitive of the building occupancy pathways. For the smallest building hot spot studied (100 cm²), the area factors were approximately 1100 for each of the radionuclides. A Bayesian statistical approach for assessing the acceptability of hot spots is proposed. A posterior distribution is generated based on the final status survey data that provides an estimate of the 99^th percentile of the contaminant distribution. Hot spot compliance is demonstrated by comparing the upper tolerance limit——defined as the 95% upper confidence level on the 99^th percentile of the contaminant distribution in the survey unit—with the DCGL_99th value. The DCGL_99th is the hot spot dose limit developed using the dose modeling research to establish area factors mentioned above. The proposed approach provides a hot spot assessment approach that considers hot spots that may be present, but not found. Examples are provided to illustrate this approach.